How InGaAs islands form on GaAs substrates: the missing link in the explanation of the Stranski-Krastanow transition

The structure of epitaxial InGaAs / GaAs layers during early stages of growth, just at the onset of islanding, is investigated ex-situ by atomic force microscopy and transmission electron microscopy. Nano-scale compositional measurements of uncapped islands by electron energy-loss imaging in cross-section allow the quantification of In gradients in the islanded layers. The In composition at the island apices is shown to be more than twice the nominal composition, while In depletion of the surrounding flat regions to a similar degree is observed. These lateral compositional inhomogeneities occur as soon as a sufficiently In enriched surface layer has been built up and are considered the driving force for the Stranski-Krastanow transition.